1. Field of the Invention
The present invention relates to a charged particle beam writing method and a charged particle beam writing apparatus.
2. Background Art
Recently, along with the development of higher levels of integration in semiconductor devices, the dimensions of the individual component devices have decreased and so has the width of wires and gates making up these components.
Photolithographic techniques which help achieve such miniaturization include the following sequential processes: applying a resist material to the surface of the substrate to be processed to form a resist film; irradiating the substrate with light or an electron beam to expose a predetermined resist pattern to form a latent image; heating the substrate as necessary; developing the pattern to form a micropattern; and etching the substrate using this micropattern as a mask.
In photolithography, the minimum width of a wiring pattern, etc. that can be resolved is proportionally dependent on the wavelength of the exposure light. Therefore, as one means of allowing miniaturization of patterns, effort has been made to reduce the wavelength of the exposure light used to form the above resist pattern latent image. Further, the development of electron beam lithography, which serves as a higher resolution exposure technique, has also been in progress. This technique inherently provides a superior resolution, since it uses electron beams, which are charged particle beams. Further, electron beam lithography is also advantageous in that great depth of focus is obtained, which enables dimensional variations to be reduced even when a large step feature is encountered. For this reason, the technique has been applied to the development of state-of-the-art devices typified by DRAM, as well as to the production of some ASICs. Further, electron beam lithography is widely used in the manufacture of masks or reticles used as original artwork for transferring an LSI pattern to the wafer.
Japanese Laid-Open Patent Publication No. 9-293670 (1997) discloses a variable shape electron beam writing apparatus used for electron beam photolithography. Such apparatus prepares pattern writing data by using design data (CAD data) of a semiconductor integrated circuit designed by a CAD system and processing it, such as correcting the data and dividing the pattern. For example, the division of the pattern into pattern segments is made on the basis of the maximum shot size, which is defined by the size of the electron beam. After this division of the pattern, the apparatus sets the coordinate positions and size of each shot and the radiation time. Pattern writing data is then produced which is used to shape the shot in accordance with the shape and size of the pattern or pattern segment to be written. The pattern writing data is divided on the basis of strip-shaped frames (or main deflection regions), and each frame is divided into sub-deflection regions. That is, the pattern writing data for the entire chip has a hierarchical data structure in which data of each of a plurality of strip-shaped frames, which correspond to the main deflection regions, is divided into a plurality of pieces of data each representing one of the plurality of sub-deflection regions (smaller in size than the main deflection regions) in the frame.
The sub-deflector scans the electron beam over the sub-deflection regions at higher speed than the main deflection regions; the sub-deflection regions are generally the smallest writing fields. When writing on each sub-deflection region, the shaping deflector forms a shot of a size and shape corresponding to the pattern or pattern segment to be written. Specifically, the electron beam emitted from the electron gun is shaped into a rectangular shape by the first aperture and then projected to the second aperture by the shaping deflector, resulting in a change in the shape and size of the beam. The electron beam is then deflected by the sub-deflector and the main deflector and directed onto the mask placed on the stage, as described above.
Incidentally, irradiating the mask with the electron beam results in generation of reflection electrons. These generated reflection electrons impinge onto the optical system, detectors, etc. in the electron beam writing apparatus, and as a result, charges are built up, thereby generating a new electric field. This changes the path of the electron beam that has been deflected toward the mask, resulting in displacement of the beam impinging position from the desired target position on the mask, which is referred to as “beam drift.” Although other problems can cause beam drift, in any case it is necessary to make corrections to cause the beam to impinge at the desired location by detecting the reference mark position on the stage in the middle of the writing operation and determining the amount of beam drift.
Conventional methods make the above corrections or calibrations at predetermined time intervals. This means that in order to reduce the amount of displacement of the electron beam impinging position due to drift, it is effective to reduce the time intervals at which the drift compensation is made. In this case, however, a reduction in the throughput results. To overcome this problem, the compensation intervals may be shortened at the start and end of the writing operation, at which there is a great change in the amount of drift. However, this means that the traveling speed of the stage is not constant, making it difficult to estimate the time of completion of the writing operation.
On the other hand, Japanese Laid-Open Patent Publication No. 9-260247 (1997) discloses a method of drift compensation including: dividing the pattern into a plurality of regions based on the allowable displacement range of the electron beam over these regions; determining the largest rate of change of displacement of the electron beam due to drift over these regions; and determining, based on the above largest rate of change and the above allowable displacement range and for each of the plurality of regions, the time intervals at which to make a drift compensation.
The method disclosed in this publication measures the amount of drift in one operation and then estimates the amount of drift and its direction that will occur in the next measurement operation and corrects the electron beam impinging position accordingly. It will be noted that this estimation is easy if the areal density of the pattern to be written is constant, i.e., the area of the portion of the pattern in each individual writing region is equal. However, the estimation is difficult if the areal density of the pattern varies from one writing region to another. However, the estimation is difficult if the areal density of the pattern varies. The above publication only discloses a method of determining the time intervals at which to make a drift compensation for each writing region in the pattern writing field and therefore fails to provide a technique for improving the writing accuracy of the electron beam writing apparatus.
The present invention has been made in view of the above problems. It is, therefore, an object of the present invention to provide an electron beam writing method and an electron beam writing apparatus capable of writing with high accuracy by improving the accuracy of the drift compensation while preventing a reduction in the throughput.